The intervertebral disc is an anatomically and functionally complex joint. The intervertebral disc is composed of three component structures: (1) the nucleus pulposus; (2) the annulus fibrosus; and (3) the vertebral endplates. The biomedical composition and anatomical arrangements within these component structures are related to the biomechanical function of the disc.
The spinal disc may be displaced or damaged due to trauma or a disease process. If displacement or damage occurs, the nucleus pulposus may herniate and protrude into the vertebral canal or intervertebral foramen. Such deformation is known as herniated or slipped disc. A herniated or slipped disc may press upon the spinal nerve that exits the vertebral canal through the partially obstructed foramen, causing pain or paralysis in the area of its distribution.
To alleviate this condition, it may be necessary to remove the involved disc surgically and fuse the two adjacent vertebra. In this procedure, a spacer is inserted in the place originally occupied by the disc and it is secured between the neighboring vertebrae by the screws and plates/rods attached to the vertebra. Despite the excellent short-term results of such a “spinal fusion” for traumatic and degenerative spinal disorders, long-term studies have shown that alteration of the biomechanical environment leads to degenerative changes at adjacent mobile segments. The adjacent discs have increased motion and stress due to the increased stiffness of the fused segment. In the long term, this change in the mechanics of the motion of the spine causes these adjacent discs to degenerate.
To circumvent this problem, an artificial intervertebral disc replacement has been proposed as an alternative approach to spinal fusion. Although various types of artificial intervertebral discs have been developed to restore the normal kinematics and load-sharing properties of the natural intervertebral disc, they can be grouped into two categories, i.e., ball and socket joint type discs and elastic rubber type discs.
Artificial discs of ball and socket type are usually composed of metal plates, one to be attached to the upper vertebra and the other to be attached to the lower vertebra, and a polyethylene core working as a ball. The metal plates have concave areas to house the polyethylene core. The ball and socket type allows free rotation between the vertebrae between which the disc is installed and thus has no load sharing capability against the bending. Artificial discs of this type have a very high stiffness in the vertical direction, they cannot replicate the normal compressive stiffness of the natural disc. Also, the lack of load bearing capability in these types of discs causes adjacent discs to take up the extra loads resulting in the eventual degeneration of the adjacent discs.
In elastic rubber type artificial discs, an elastomeric polymer is embedded between metal plates and these metal plates are fixed to the upper and the lower vertebrae. The elastomeric polymer is bonded to the metal plates by having the interface surface of the metal plates be rough and porous. This type of disc can absorb a shock in the vertical direction and has a load bearing capability. However, this structure has a problem in the interface between the elastomeric polymer and the metal plates. Even though the interface surfaces of the metal plates are treated for better bonding, polymeric debris may nonetheless be generated after long term usage. Furthermore, the elastomer tends to rupture after a long usage because of its insufficient shear-fatigue strength.
Because of the above described disadvantages associated with either the ball/socket or elastic rubber type discs, there is a continued need for the development of new prosthetic devices.